The present invention is directed to the field of filtration systems. More specifically, the present invention is directed to a filtration system combining cross-flow currents and secondary flow currents such as Dean-Flow currents to assist in surface cleaning of the filter unit.
The art has seen various filtration devices employing different methods for removing particulate or impurities from a feed fluid. For example, so-called dead end filtration systems force all of the feed fluid through a filter to separate impurities therefrom. Dead end filters designs may place a filter either directly across a flowpath or at an oblique angle to the flowpath. U.S. Pat. No. 1,822,006 to Bull discloses a dead-end filter wherein the feed fluid enters a cylindrical chamber housing a cylindrical filter and follows a spiral flowpath from one end of the cylindrical filter to the other. All of the feed fluid eventually forced through the cylindrical filter as the fluid flowpath terminates adjacent the far end of the filter from the input end. A sump chamber is provided below the helical fluid flowpath and filter for collecting material separated by the filter component. As all of the fluid must pass through the filter and as the sump chamber is not open throughout operation, the velocity of the fluid about the filter will continuously decrease to zero unless the filter is cleared. U.S. Pat. No. 3,637,078 to Hollar discloses an oil filter having a spiral guide positioned about a pleated filter. The Hollar filter is another example of a dead-end filter design as all of the oil entering the filter must pass through the filter cartridge. Particulate and other contaminants collect on the filter surface within the expanses spanning between the adjacent pleats and the spiral guide.
Other filter designs employ a spiral fluid flowpath to separate heavier particulate from the fluid medium. These centrifugal particulate separation devices employ a spiral path to generate centrifugal forces which force the heavier particulate to the outside of the spiral flowpath. For example, U.S. Pat. No. 3,402,529 to Franz provides a spiral flowpath down along a cylindrical non-porous sleeve having a significantly wider mouth portion at one end. The fluid medium air flows within the spiral path to the inside of the flow of the heavier contaminants. The Franz filter collects the separated particles at the bottom of the unit opposite the open mouth portion which then acts as an intake for the air. The Franz design is impractical for applications where the fluid medium is a liquid such as a fuel, oil, or water, however, as many types of colloidal particulate are known which have a lower specific gravity than the fluid medium. Such lighter particulate will tend to be forced to the inside of the spiral path by the heavier fluid medium. These lighter particulate may collect in the filter itself and require the filter unit to be shut down and the filter replaced or cleaned.
Cross-flow filtration is yet another alternative method for filtering particulate from a fluid medium. Cross-flow filtration differs from dead-end filtration in that the feed fluid provided to the filter unit actually passes across the enclosed filter membrane or filter media. Cross-flow filtration describes the condition of fluid flow past a membrane while the fluid is being pressurized against the surface. Cross-flow filtration performance has been found to be governed the pore size of the filter media, the generated fluid shear force across the surface of the filter media, and the deposit layer and the control of the deposit layer formation. Only a portion of the feed fluid passes through the filter to become filtrate, or permeate, fluid. The other portion of the feed fluid continues past the filter media and exits the filter unit as concentrate, or retentate, fluid. Flow velocity is of fundamental importance to the performance of a cross-flow filter. Should the flow velocity across the surface of a filter media become zero, the cross-flow ceases and the dead-end filtration begins. Additionally, the cake which forms on the filter media at zero velocity becomes thicker as the flow velocity, parallel to the medium, decreases. The thickness of the cake layer in a flow channel is determined by the shear force on the membrane surface which is roughly in direct proportion to the feed viscosity and the feed flow velocity. Therefore, higher fluid velocity entails a thinner deposit layer, a lower hydraulic resistance, and a higher filtrate flux.
In almost every filtration process a xe2x80x98secondary membranexe2x80x99, also called a xe2x80x98dynamic membranexe2x80x99 will be created. The contaminants which constitute the secondary membrane first fill up the pores and then form a very thin cake of constant thickness. The transition time for pore filling may be very short. Particulate has been observed on first use of a filter to immediately enter into the pores of the filter media, although only to a limited extent. The result is that a cross-flow filtration system typically experiences a rapid flux drop at the beginning of its use for filtration. Thereafter, the flux is stabilized at a relatively satisfactory level, and remains almost constant with a very slow decline as the process continues. This is unlike dead-end filtration where the flux rate drops continuously from the time the filter is operated until complete clogging. The rate of flux drop depends on the selection of membrane pore size and the nature of the contaminants. Filter pore-size must therefore be selected with a view towards the expected contaminants in order to control the formation of the deposit layer.
It is further known that as fluid flows through a curved channel about a normal or longitudinal axis that a secondary flow, which is the flow perpendicular to the main direction of flow, occurs. The secondary flow phenomenon is caused by centrifugal force which forces the fast moving fluid in the channel core toward the outside wall from where it journeys back along the floor and roof of the channel to the inside wall. When the fluid is forced through the channel at a critical velocity, a double-vortex flow known as Dean-Flow currents is formed.
The phenomenon of Dean-Flow was first observed by W. R. Dean who studied the secondary flow created by the motion of fluid in a curved pipe. Flow in a curved channel appears unstable for small disturbances, compared with a sudden increase in the loss of head when flow passes through a straight pipe at a critical velocity, i.e., the transition from laminar to turbulent flow. No such sudden increase in the loss of head is generally observed in a pipe of significant curvature, even though flow rate is much higher than the critical flow rate. This phenomenon suggests that the pressure drop is much smaller in a curved pipe than in a straight pipe at the same flow rate. The flow in a curved channel has been characterized as a double vortex flow, as shown in FIG. 3. The Dean number, K, is the characteristic parameter used to describe the formation of vortices in this situation:
K=(vxc2x7d/"ugr")xc2x7(d/R)0.5
Where v is the tangential velocity of the fluid, d is the diameter of the pipe, R is the radius of the pipe curvature, and "ugr" is the kinematic viscosity of the fluid. The higher the Dean number, the stronger the vortices induced.
Early studies on this secondary flow phenomenon were mainly focused on the heat transfer in a coiled heat exchanger. These studies showed that the heat transfer coefficient was much higher for a curved pipe than for a straight pipe. In recent years, studies on the double-vortex secondary flow show that the secondary flow may be employed to greatly reduce the filtered material concentration polarization in filters. As the fluid spins in a curved channel, a control mass of fluid travels radially, eventually reaching the outer wall where it must change direction towards a return path. The resulting flow profile takes the form of a toroidal vortex in which a fluid particle moves in three dimensions. The vortex profile generates a high shear rate which acts to transport material, or debris, away from the membrane surface. Such a vortex flow exists in both the laminar and turbulent flow regions and the vortex structure persists up to 1000 times the critical flow rate.
The prior art has seen different methods for creating and employing secondary flow currents to enhance the performance of pressure-driven filtration processes. For example, U.S. Pat. No. 5,626,758 to Belfort discloses a filtration device employing a wound helical membrane tube. Such a device, however, cannot be practicably employed in a back-flush filtration system in which back pressure is applied to the permeate side the filter media. Back-flushing allows a reverse fluid flow to penetrate the filter media and clear any plugged pores so as to recover the decreased permeate flux rate. Periodic back-flushing is essential for challenging processes, e.g., those involving colloidal solids which are able to extrude into the pores of the filter media, so as to maintain an acceptable permeate flux rate across the filter media. Back-flushing provides a desirable alternative to shutting down the filter unit for filter media replacement.
U.S. Pat. No. 5,143,630 to Rochigo discloses a rotary disc device having spiral grooves formed on one face to generate Dean Flow vortices across. Additionally, U.S. Pat. No. 4,790,942 to Shmidt discloses a rotating cylinder filter for generating Taylor vortices within an annular gap outside the rotating cylinder. Both of these devices actively rotate the filtration components to establish the secondary flow currents for enhanced filter performance. The need for moving parts, however, can increase the maintenance burden on their users. Moreover, the Shmidt device may not be suitable for scale-up to high capacity applications due to the difficulty of bundling multiple rotating cylinders while maintaining the critical dimension of the annular gaps. Additionally, sealing fluid in multiple rotating shafts would be impractical.
While cross-flow filtration is a step improvement over dead-end filtration, an inherent weakness common to all traditional cross-flow filtration designs frustrates its operation: a pressure drop in the fluid passageway exists from the feed inlet to the concentrate outlet. The pressure drop results in a non-uniform pressure differential through the filter membrane across the entire length of the fluid passageway. As the pressure differential across the filter membrane is highest near the inlet end of a cross-flow filter, fouling of the filter membrane tends to occur there. The present invention solves this problem of fouling at a filter inlet by generating a secondary flow to assist in clearing away the particulate fouling the cross-flow filter.
The strength of the vortex action created by the secondary flow is directly proportional to the flow rate and the geometry of the curved channel. As the feed flow has the highest tangential velocity at the inlet, the vortices are the strongest there. The present invention contemplates that the stronger vortices at the inlet may be employed to compensate for the higher incidence of fouling which occurs at the inlet end of cross-flow filtration units. Overall, the secondary flow discussed may be employed to not only reduce the pressure drop along a filter, but also to enhance the shear force along the surface of the filter media to thereby reduce particulate build-up.
There is therefore a need for a cross-flow filtration system which can minimize the effects of the pressure drop along the filter membrane. There is also a need for a self-cleaning filter unit which is able to conduct colloidal particles having a lower specific gravity than the fluid medium away from a filter membrane. There thus exists, then, a need in the art, for a cross-flow filtration device having still further improved surface cleaning capabilities and which may be scaled down to occupy a small space for applications having limited available space for operation.
In view of the needs of the art, the present invention provides a cross flow filtration assembly which develops secondary flow currents in a fluid flowing in a spiral flowpath about a generally cylindrical filter media. Desirably the secondary flow currents developed by the spiral flowpath are Dean Flow currents. Dean Flow currents describe a particular flow regime developed for a spiraling fluid flowpath when fluid is forced therethrough at a critical flow velocity. Dean Flow currents are developed in opposing pairs of corkscrew vortices which travel along the spiral fluid flowpath and provide a shear cleaning current across the filter media surface so as to conduct away particles entrapped by the filter media. Fluid flowing through a spiral flowpath at less than the Dean Flow critical velocity will not develop the opposing corkscrew currents therein while fluid flowing too quickly through a spiral flowpath degenerates into a purely turbulent flow regime. Dean Flow currents have been demonstrated to better maintain the flux rate across a filter media so as to extend the operating period of a filter unit between required backflushing or maintenance.
The present invention provides a cross-flow filtration assembly including a filter housing having an elongate housing wall having opposed first and second open ends and an elongate cylindrical interior surface defining a housing cavity. A housing cap wall extends across the first open end of the housing wall, and a housing base wall extends across the second open end of the housing wall. The filter housing further defines an input feed port, a permeate output port, and a retentate output port, all in fluid communication with the housing cavity. An elongate porous filter is mounted within the housing cavity. The filter defines an open first end, an opposed closed second end, and an elongate cylindrical permeate passageway extending therebetween. The permeate passageway extends in fluid communication with the permeate output port through the first open end of the filter. The filter includes a substantially cylindrical outer filter surface, wherein the outer filter surface and the interior surface of the housing wall define an elongate annular gap therebetween. A spiral guide extends through the annular gap between the outer filter surface and the interior surface of the housing wall so as to define a fluid flow passage extending between the input feed port and the retentate output port. Fluid enters the filter assembly through the input feed port and into the fluid flow passage substantially along a tangential flowpath along said filter. The pitch and width of the spiral define a cross-sectional area for the fluid flow passage which, for the velocity of the fluid flowing therethrough, induces secondary flow currents in the fluid as it travels along the spiral fluid flow passage. Desirably, the filter unit develops Dean Flow currents through the spiral fluid flow passage.
The present invention also contemplates a filter unit employing a number of such cross-flow filtration assemblies. The filter unit includes an elongate cylindrical filter housing having an interior cylindrical wall defining an elongate filter cavity, a tangential feed inlet in fluid communication with the filter cavity extending substantially tangential to the interior cylindrical wall, and a retentate outlet port in fluid communication with the filter cavity. An elongate cylindrical filter cartridge supported in the filter housing includes an elongate porous cylindrical filter wall having an elongate cylindrical outer filter surface and an elongate inner filter surface defining a permeate passageway. One end of the cartridge defines a permeate outlet port in fluid communication with the permeate passageway and the opposite end of said cartridge is closed by a non-porous filter cap. A spiral fluid guide spans between said interior cylindrical wall of the filter housing and the outer filter surface. The spiral guide defines a spiral fluid passageway extending between the feed inlet and the retentate outlet port. The spiral fluid passageway imparts a secondary flow current to fluid flowing therethrough. The spiral fluid passageway may be of dimension so as to impart Dean-Flow currents to fluid flowing therethrough.
The filter unit of the present invention may further include a filter unit housing defining a filter unit housing cavity and a fluid feed port, a concentrate outlet port, and a filtrate outlet port in fluid communication with said filter unit housing cavity. A plurality of the cross-flow filtration assemblies are mounted within the housing cavity whereby for each of the plurality of cross-flow filtration assemblies the tangential feed inlet is in fluid communication with the fluid feed port of the filter housing and the retentate outlet port is in fluid communication with the concentrate outlet port of the filter housing. The permeate passageway is similarly in fluid communication with the filtrate outlet port of said filter housing
The present invention also provides a method of filtering particles from a fluid. The method includes the step of providing a filtration assembly having an interior cylindrical filter, an outer filter housing concentrically supported about the interior cylindrical filter so as to define an annular fluid cavity therebetween, and a spiral guide spanning between the filter and the housing so as to define a spiral fluid passageway from one end of the filter to the opposed end of the filter. The method of the present invention then includes the step of forcing a feed fluid having particulate matter suspended therein through the fluid passageway at a velocity sufficient to induce secondary flow currents in the fluid so as to provide a shear cleansing current across the filter. The induced secondary flow currents may take the form of Dean Flow currents.
The present invention thereby provides a filtration assembly and filter unit having applications in reverse osmosis, nano-filtration, ultra-filtration, micro-filtration, and screen mesh, or particle, filtration applications.